Magnetic field sensors that sense the rotational motion of a target are known. The target can be a hard ferromagnetic (permanent magnet) or a soft ferromagnetic target. Magnetic field sensors that detect the features of a ferromagnetic gear target belong to a class of sensors known as “gear tooth sensors.” Gear tooth sensors are key elements in engine management applications, anti-lock braking systems, transmission systems, and other industrial or automotive systems.
In this type of magnetic field sensor, the magnetic field associated with the target's mechanical profile is sensed by a magnetic field sensing element, such as Hall element or magnetoresistive (MR) element. In the case where the target is a soft ferromagnetic material a permanent magnet may be positioned near the sensor to provide a magnetic field to the sensing element. As the target's features pass the sensing element, the magnetic field experienced by the sensing element varies in relation to the target profile. The sensing element provides a signal proportional to the sensed magnetic field. The sensor processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses a threshold. Such an output can be used to provide rotational speed information.
Some sensors, referred to as differential sensors, contain at least one pair of sensing elements configured in a differential arrangement. In differential magnetic field sensors, the difference between the signals provided by the two sensing elements is used to generate a differential magnetic field signal indicative of transitions in the target's features. The differential sensor processes the differential magnetic field signal to generate an output signal that switches states each time the differential magnetic field signal crosses a threshold. Thus, detection of the approach and retreat of each feature of a rotating ferromagnetic gear results in an output signal that is typically a square wave representation of rotation of the ferromagnetic gear.
Various types of threshold-based mechanisms can be used to generate the output signal. One such mechanism is referred to as a threshold detector or peak-to-peak percentage detector (and referred to herein as a threshold detector). It uses thresholds defined as a percentage of the peak-to-peak magnetic field signal (e.g., 40% and 60%). One type of threshold detector is described in U.S. Pat. No. 5,917,320 entitled DETECTION OF PASSING MAGNETIC ARTICLES WHILE PERIODICALLY ADAPTING DETECTION THRESHOLD, which is assigned to the assignee of the subject application. Each threshold is determined based on a previous negative and positive peak. If the magnetic field signal doesn't cross one of these thresholds (e.g., because of a sudden large signal shift), the thresholds cannot be updated. If the thresholds are not updated, the output signal will stop switching.
Another mechanism, referred to as a slope-activated, peak-referenced or peak detector (and referred to herein as a peak detector), uses a fixed or variable threshold referenced to the last positive or negative peak (i.e., the last peak or valley) of the magnetic field signal. In the peak detector, the threshold differs from the positive and negative peaks of the magnetic field signal by a predetermined amount. In this type of detector, the output signal changes state when the magnetic field signal comes away from a peak or valley by the predetermined amount. One type of peak detector is described in U.S. Pat. No. 6,091,239 entitled DETECTION OF PASSING MAGNETIC ARTICLES WITH A PEAK-REFERENCED THRESHOLD DETECTOR, which is assigned to the assignee of the subject application. Another example of a peak detector can be found in U.S. Pat. No. 6,693,419 entitled PROXIMITY DETECTOR, also assigned to the assignee of the subject application.
In order to accurately detect the passing features of the rotating ferromagnetic object, these circuits must be capable of closely tracking the magnetic field signal. Typically, one or more digital-to-analog converters (DACs) are used to generate a DAC signal, which tracks the magnetic field signal. In some architectures, one DAC is used to track the positive peaks of the magnetic field signal (PDAC) and another DAC is used to track the negative peaks of the magnetic field signal (NDAC). In other architectures, a single DAC tracks both the positive and negative peaks of the magnetic field signal.
Both detection schemes have their advantages and disadvantages. The peak detector tends to offer a more robust approach to switching, with guaranteed switching even in the presence of large signal variation. The peak detector is commonly used for sensors in automotive anti-lock braking systems (ABS) and transmission applications where magnetic anomalies may compromise the functionality of the sensor. The threshold detector, on the other hand, tends to provide greater switching accuracy.
Some gear tooth sensor architectures incorporate both threshold and peak detection in order to differentiate vibration from true rotation. Examples include the ATS651LSH, ATS655/7LSH and ATS692/3LSH family of differential Hall-effect gear tooth sensors available from Allegro Microsystems, Inc.